1. Field of the Invention
This invention relates to a center pivot irrigation system having the ability to irrigate the corners of the field being irrigated. More particularly, this invention relates to an improved means for controlling the distribution of water from the corner span.
2. Description of the Related Art
Conventional center pivot irrigation systems comprise an elongated main boom pivotally connected at its inner end to a center pivot structure and extending outwardly therefrom. The main boom of the conventional center pivot irrigation system is comprised of a plurality of pipes connected together in an end-to-end fashion and which are supported upon a plurality of drive towers. In most conventional center pivot irrigation systems, the outermost drive tower is the master tower with the other drive towers being selectively driven in response to angular displacement of the boom section adjacent thereto.
In the early 1970s, corner systems for center pivot irrigation systems were developed to enable the corners of a square field to be irrigated. See, for example, U.S. Pat. Nos. 3,797,517; 3,802,726; and 3,902,668. Corner systems usually consist of an extension boom or arm, sometimes referred to as a corner span, which is pivotally connected to the outer end of the main boom and which is supported on at least one steerable drive tower. A guidance system is provided for guiding or steering the extension tower, and extension boom, out into the corners of the field as the main arm travels around the field. Perhaps the most popular method of guiding the extension tower is the buried wire system described in U.S. Pat. No. 3,902,668.
A plurality of spaced-apart sprinklers are provided on the main boom as well as on the extension boom. The sprinklers on the extension boom are not activated until the extension boom moves out into the corners of the field. A difficulty arises in providing uniform water distribution in the corner areas of the field due to the fact that the sprinklers on the extension boom cover varying areas of the field as the extension boom moves out into the corner of the field and when the extension arm is retracting from its fully extended position.
Heretofore, groups of sprinklers on the extension boom were switched on or off, depending upon the angle the extension boom made with the main boom. This resulted in some overwatering and some underwatering, depending upon the relative position of the extension boom.